Pedals are one of the three contact points of a bike (i.e., pedals, saddle and handlebars). Depending on the type of cycling you do there are a number of options available.
Commuting/Leisure – Flat pedals in plastic or alloy (easy to stop in traffic)
Leisure/Exercise – Flat pedals with toe clips and straps (keeps your foot in correct position)
Sportif/Racing/Mountain biking – Clipless pedals (SPD, Look etc.,) are best as they are used in conjunction with cycling shoes.